Home · A-Level Biology 9700 · Nucleic acids and protein synthesis · The structure of nucleic acids · Practice
AS · Practice questions · The structure of nucleic acids

Count the rings, pair the bases.

Six original Cambridge-style questions on the structure of nucleotides, DNA and RNA, from base percentages to comparing DNA with mRNA.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Sugar, base, bond.

01
Recall
[3 marks]

Describe the structure of an ATP molecule and state how it differs from a DNA nucleotide.

  • ATP is a nucleotide made of the base adenine, the pentose sugar ribose and three phosphate groups. ✓
  • A DNA nucleotide has deoxyribose instead of ribose. ✓
  • A DNA nucleotide has only one phosphate group (and could have any of four bases). ✓
02
Explain
[3 marks]

Explain why the two strands of a DNA double helix are described as complementary and antiparallel.

  • Complementary: each base pairs only with one other (A with T, C with G), so the sequence of one strand determines the sequence of the other. ✓
  • Antiparallel: the strands run in opposite directions. ✓
  • One strand runs 5′ to 3′ and the other 3′ to 5′, so the 5′ end of one lies opposite the 3′ end of the other. ✓
03
Calculation
[2 marks]

Analysis of a double-stranded DNA sample shows that 29% of the bases are thymine. Calculate the percentage of cytosine.

  • T = A = 29%, so A + T = 58% and C + G = 42%. ✓
  • C = G, so cytosine = 21%. ✓
04
Explain
[2 marks]

A purine always pairs with a pyrimidine in DNA. Explain why this is important for the structure of the double helix.

  • A purine has a double ring and a pyrimidine a single ring, so every base pair is the same total width (three rings). ✓
  • The two sugar-phosphate backbones therefore stay a constant distance apart along the whole molecule, giving a regular helix. ✓
05
Data
[3 marks]

A length of DNA contains 200 base pairs. 35% of the bases are guanine. Calculate the total number of hydrogen bonds between the two strands.

  • G = C = 35%, so C-G pairs = 70% of 200 = 140 pairs; A-T pairs = 60. ✓
  • Hydrogen bonds = (140 × 3) + (60 × 2) = 420 + 120. ✓
  • Total = 540 hydrogen bonds. ✓
06
Explain
[4 marks]

Compare the structure of a DNA molecule with the structure of an mRNA molecule.

  • Both are polynucleotides with a sugar-phosphate backbone joined by phosphodiester bonds; both contain A, C and G. ✓
  • DNA is double-stranded (a double helix with hydrogen-bonded base pairs); mRNA is single-stranded. ✓
  • DNA contains deoxyribose and thymine; mRNA contains ribose and uracil. ✓
  • DNA is very long (many genes); mRNA is short, a copy of one gene. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.